(a) Field of the Invention
The present invention relates generally to a pre-preprocessing method for digital images. More particularly, it relates to a digital pre-processing method that iteratively distorts pixels in an image to minimize perceived blockiness following a digital compression and decompression cycle.
(b) Description of Related Art
Digital image compression methods selectively quantize image information to reduce the amount of information needed to reconstruct a reasonable approximation of the original image. Quantization compacts image information, but it inherently sacrifices detail and the overall perceived image quality.
There are a variety of existing compression standards for digital images and video. For example, JPEG, H.263, MPEG-1, MPEG-2, and MPEG-4 all use a block-based discrete cosine transform (DCT) compression method. Block-based DCT methods sub-divide an original image into a matrix of pixel blocks. Typically, each block contains 64 pixels (8xc3x978), and each pixel has a value between 0 and 255. Each block is represented by a vector of 64 DCT coefficients ranging in value from 0 to 255.
Digital compression methods seek to quantize and thereby reduce the number of coefficients needed to reconstruct an approximation of an original image. For example, the range of coefficient values may be reduced to values between 0 and 15 to provide a four-fold reduction in the information needed to describe an image. Quantized coefficient values may be predetermined by the compression standard, or may vary for each compressed image.
Blocking artifacts are a highly objectionable type of distortion that result from using standard digital image compression methods. Blocking artifacts arise from discontinuities on the boundaries between pixel blocks in the reconstructed image. Image blockiness results when the pixel block border discontinuities are large enough to prevent a perceived seamless blending of adjacent blocks, and the reconstructed image appears to be an assembly of discrete blocks. Blockiness is most pronounced within the uniform regions of a reconstructed image because the human eye can discriminate extremely small discontinuities in an otherwise uniform image field.
A number of post processing methods are available to reduce the blockiness of images that have been reconstructed (decompressed) from compressed information. These methods typically apply a digital filter to the image that smooths pixel block border discontinutites. However, post processing methods are disadvantageous because they require modification of the decompression method at the individual users"" systems.
A few commercially available MPEG compression programs use pre-processing digital filtering methods to reduce spatial and temporal noise in the original image. Although such pre-processing compression methods can improve reconstructed image quality for a given compression ratio, they do not work to minimize blockiness.
Thus, a need exists for a pre-processing method of reducing the blockiness of images that are reconstructed using standard digital image compression formats and methods.
In accordance with one aspect of the present invention, a method for reducing blocking artifacts associated with compression of a digital image comprises steps of segmenting the image, mapping pixel blocks, comparing pixel values, adjusting pixel values, and replacing pixel values. An original image is segmented into pixel blocks. The pixel blocks are mapped into a predetermined set of images associated with an image compression format to produce a second image. Adjacent border pixels within the second image are compared and adjusted to produce a third image. Non-border pixels within the third image are replaced with pixels from the original image to produce a fourth image.
In accordance with another aspect of the present invention, a method for reducing blocking artifacts associated with compression of a digital image comprises steps of segmenting the image, calculating a blockiness measure, mapping pixel blocks, comparing pixel values, replacing pixel values, and repeating the steps until the blockiness measure crosses a predetermined level. An original image is segmented into pixel blocks. The pixel blocks are mapped into a predetermined set of images associated with an image compression format to produce a second image. A blockiness measure is calculated for the second image, and the blockiness measure is compared to a predetermined value. Adjacent border pixels within the second image are compared and adjusted to produce a third image. Non-border pixels within the third image are replaced with pixels from the original image to produce a fourth image. The steps of mapping, comparing, adjusting, and replacing are repeated until the blockiness measure crosses a predetermined threshold.
The invention itself together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.